1. Technical Field
The present invention relates to a surface illumination device. Specifically, the present invention relates to a surface illumination device, which is used in display while incorporated in mobile devices such as a smartphone and a tablet computer.
2. Related Art
In the smartphone, the tablet computer, an electronic book reader, and the like, an icon is lighted in a position of a switch, and the position and a type of the switch is expressed by the icon. For example, in a smartphone 11 in FIG. 1(A), an icon 13 is displayed below a liquid crystal display screen 12. When the icon 13 is pressed by a finger, a switch 14 provided beneath the icon 13 is turned on to switch a function of the smartphone 11.
FIG. 1(B) illustrates a surface illumination device 15 for the display, which is incorporated in a lower portion of the smartphone 11 in FIG. 1(A). FIG. 2(A) illustrates a schematic section of the surface illumination device 15. The surface illumination device 15 includes a light guide plate 16 and a light source 17. The light guide plate 16 is molded using a transparent material, such as a polycarbonate (PC) resin and a polymethylmethacrylate (PMMA) resin, which has a high refractive index. The light source 17 is a minute light source (a point light source) in which an LED is used, and the light source 17 is disposed while a light-exit window is opposed to one (light incident end face 16a) of end faces of the light guide plate 16. Many minute prism deflection patterns 18 are formed in a lower surface (sometimes an upper surface) of the light guide plate 16, and an icon-shaped display unit 19 is constructed by a set of deflection patterns 18 as illustrated in FIG. 2(B). The deflection patterns 18 are arrayed in an arc shape about a point near the light source 17 and extend in directions along the arc about the point. A thin, flexible light guide sheet is used as the light guide plate 16 in the case that the switch 14 is disposed beneath the display unit 19. Although the icon 13 and the display unit 19 are expressed by a character “A” in FIGS. 1(A) and 1(B), actually design marks such as a magnifying glass and a handset are frequently used as illustrated in FIG. 3. For the sake of convenience, the icon 13 and the display unit 19 are expressed by the character “A” except FIG. 3.
When the light source 17 emits light in the surface illumination device 15, the light incident to the light guide plate 16 from the light incident end face 16a is guided in the light guide plate 16 while totally reflected by the upper surface, the lower surface, and both side surfaces of the light guide plate 16. When the light guided in the light guide plate 16 reaches the display unit 19 as illustrated in FIG. 2(A), the light is totally reflected by a deflection reflecting surface 18a of the deflection pattern 18. In the light totally reflected upward by the deflection reflecting surface 18a, the light incident to the upper surface (a light-emitting surface 16c) of the light guide plate 16 at an angle smaller than a total reflection critical angle is transmitted through the light incident end face 16a to emit upward (the light is transmitted while refracted by the deflection pattern 18 in the case that the deflection pattern 18 is provided in the light-emitting surface 16c). As a result, the light emits into the icon shape to light the icon 13 of the smartphone 11.
However, in the smartphone 11 in which the surface illumination device 15 is used, when a user vertically inclines the smartphone 11 while holding the smartphone 11 in the hand, unfortunately luminance of the icon 13 changes rapidly, or the icon 13 is seen while flickering. The reason the phenomenon is generated will be described with reference to FIGS. 4 and 5. FIGS. 4(B) and 5(B) illustrate directional patterns of the light emitting from the light-emitting surface 16c of the light guide plate 16 when the directional pattern is viewed from a direction (a Z-direction) perpendicular to the light-emitting surface 16c, an X-direction indicates a length direction (a crosswise direction of the smartphone 11) of the light guide plate 16, and a Y-direction indicates a width direction (a longitudinal direction of the smartphone 11) of the light guide plate 16.
In the case that the light emitted from the small light source 17 is totally reflected by the deflection pattern 18 to emit from the light-emitting surface 16c, because the light headed from the light source 17 to certain deflection pattern 18 is the light having a narrow range when viewed from the direction perpendicular to the light-emitting surface 16c, the light emitting from the light-emitting surface 16c also has the narrow directivity. That is, light La that goes straight to reach the display unit 19 or the deflection pattern 18 as illustrated in FIG. 4(A) has a directional pattern Ca that is lengthened in the X-direction as illustrated in FIG. 4(B). Therefore, although the light having the directional pattern Ca can be recognized with high luminance from the front direction of the smartphone 11, the light rapidly becomes dark to hardly recognize the icon 13 when the smartphone 11 is vertically inclined.
In the light incident to the display unit 19, part of the light is incident to the deflection pattern 18 after totally reflected by a side surface 16b of the light guide plate 16 like light Lb in FIG. 5(A). As indicated by a broken line in FIG. 2(A), the light Lb is obliquely incident to the deflection pattern 18 and totally reflected toward the oblique direction by the deflection pattern 18. Therefore, the light Lb emitting from the light-emitting surface 16c has a narrow directional pattern Cb that is inclined as illustrated in FIG. 5(B).
In the case that the light totally reflected by the side surface 16b of the light guide plate 16 has a high intensity, because the light Cb having the directional pattern Cb is added to the light La having the directional pattern Ca as described above, the icon 13 repeatedly becomes bright and dark to flicker, and the icon 13 is hardly seen, when the user vertically inclines the smartphone 11 while holding the smartphone 11 in the hand.
In a surface illumination device 21 in FIGS. 6(A) and 6(B), in the case that the display unit 19 is constructed by a set of dome-shaped pattern 22 instead of the prism deflection pattern 18, because the light incident to the display unit 19 from the light source side as illustrated in FIG. 7(A) is scattered by the dome-shaped pattern 22, the directivity of the light emitting from the light-emitting surface 16c, particularly the directivity in the Y-direction spreads to obtain a directional pattern Cc as illustrated in FIG. 7(B). Accordingly, even if the user vertically (the Y-direction) inclines the smartphone 11, the icon 13 hardly rapidly becomes dark or flickers. However, in the case that the display unit 19 is constructed by the dome-shaped pattern 22, because the light La from the light source side scatters, the luminance of the icon 13 is extremely degraded when viewed from the front.
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-243822